Transformer means for a cascadetransformer type potential divider



P 1970 c. F. WOLFENDALE 3,531,714

TRANSFORMER MEANS FOR A CASCADETRANSFORIVIEI" TYPE POTENTIAL DIVIDER Filed Aug. 18, 1967 Hal.

United States Patent 0 3,531,714 TRANSFORMER MEANS FOR A CASCADE- TRANSFORMER TYPE POTENTIAL DIVIDER Peter Caleb Frederick Wolfendale, Chiltern Close, Great Briclrhill, Bletchley, Buckinghamshire, England Filed Aug. 18, 1967, Ser. No. 661,718 Claims priority, application Great Britain, Aug. 18, 1966, 37,058/ 66 Int. Cl. H02p 13/06 US. Cl. 323-435 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to potential dividers.

It is known in the patented prior art to provide a decade potential divider which includes a transformer made by twisting together in a random fashion ten similar wires. The resulting rope is then wound onto a high permeability toroidal core, so that each wire is inductively coupled to each other wire. Because of the random nature of the rope, the coupling of each wire to each of the other wires is similar.

The primary object of the present invention is to provide a potential divider including a transformer which includes two or three similar windings extending adjacent each other for mutual inductive coupling, said windings being disposed symmetrically with respect to each other and in a regular configuration. A pair of input terminals are connected respectively to the ends of first and second ones of said windings, said first and second windings being electrically connected in series between said input terminals to provide at least one junction between said windings. The windings are so disposed that current flows through adjacent portions thereof between said terminals in the same sense, and a plurality of output terminals are connected to said input terminals and said one junction, respectively.

In accordance with the present invention, a potential divider capable of dividing a potential into a greater number of parts than two or three can be provided by connecting a plurality of such transformers in cascade. Because of the regular and symmetrical dispositions of the windings, higher accuracy can be achieved than with a random configuration, which is used where more than three windings are included in one transformer.

Another aspect of the invention provides a potential divider including a transformer which comprises at least three windings extending adjacent each other for mutual inductive coupling, a pair of terminals being connected respectively to ends of first and second windings that are electrically connected in series. The windings define at least one junction therebetween, said windings being so disposed that current flows through adjacent portions thereof between said input terminals in the same sense. Negative impedance means are connected across a third one of said windings to present, an impedance having a negative real part. The negative impedance device can be arranged to cancel, at least partially, the iron, contact and other resistive losses in the transformer.

Other features and advantages of the invention will appear from the following description of embodiments thereof, given by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a potential divider in accordance with the invention,

'FIG. 2 is a schematic diagram of a negative impedance device which can be used in the potential divider, and

FIG. 3 is a diagrammatic illustration of the manner of winding the transformer windings on a core.

The potential divider shown in FIG. 1 comprises four cascaded transformers 1, 5, 7 and 9 operable to divide an alternating potential applied across input terminals 2 and 3 into sixteen parts in accordance with different connections selectable by switches 4, 6, 8 and 10. Each of the transformers includes a separate core upon which is wound two or three wires helically twisted together to define a rope consisting of the two or three strands insulated from each other. Referring to the three-strand embodiment of FIG. 3, the three strands have identical lengths and all pursue siimlar regular paths relative to each other strand. The relative dispositions of the windings of each transformer are, as nearly as possible, identical so that the mutual inductance of each winding with each other winding of the same transformer is substantially identical.

In the illustrated embodiment of the invention, the transformer 1 comprises three windings, 11, 12 and 13. The windings 11 and 12 are connected in series in such a way that current flowing from one input terminal to the other flows in the same sense in adjacent portions of the windings 11 and 12. The switch 4 connects transformer 5 across either winding 12 or winding 11. Because the windings 11 and 12 are similar, are tightly twisted together, are wound on a common core, and are highly symmetrically disposed relative to each other, the voltage induced in each winding 11, 12 is substantially identical. The same applies to the successive transformers, which in this embodiment have only two windings each, although a third winding is used for the transformers 5, 7 and 9 also in a preferred embodiment.

The third winding such as 13 is also arranged symmetrically, and in this embodiment is used to couple a negative impedance 10 on to the core, the coupling of the negative impedance being similar to the coupling of the windings of 11 and 12. The real part of the impedance of the device 10 presented to the winding 13 is negative. The value of the negative impedance is chosen so as to cancel at least partially the losses of the core onto which windings 11, 12 and 13 are wound, and other resistive losses.

The effect of the negative impedance device 10 is to reduce the current applied to the transformer which is dissipated resistively, and hence effectively increases the input impedance of the transformer. The reactive part of the transformer input impedance may also be modified as desired.

The negative impedance may take various forms which will occur to those skilled in the art. FIG. 2 shows a preferred device, which comprises a differential amplifier :14.

The output of the amplifier 14 is connected through two similar resistors 15 and 16 in series to an input terminal 17 of the device 10, and also through a feed-back impedance 18 to another input terminal 19 of the device. The terminal 19 is connected to one input of the amplifier 14, and the other input of the amplifier is connected to the junction of resistors 15 and 16. Since resistor 15 has the same value as resistor 16 then the input impedance at terminals 17 and 19 will be -Z where Z, is the impedance value of impedance 18 if the gain of the amplifier is high. The third winding, such as 13 in FIG. 1, is connected to terminals 17 and 19 and the value of 18 is adjusted to give a suitable input impedance to the device 10.

There may be considerably more transformers in the divider than shown. Also the principle of coupling a negative impedance to the core of the transformer can be applied to transformers with more than three windings.

Where three windings are used on a transformer each as 1, all three windings can be connected in series between the input terminals, to give trinary potential division at the transformer, instead of the binary division of the embodirnent of FIG. 1.

I claim:

1. In a potential divider of the type including a plurality of transformers connected in cascade, the improvement wherein at least a first one of said transformers comprises;

at least two but no more than three similar transformer windings helically twisted together in a rope-like manner to effect mutual inductive coupling therebetween, said windings being symmetrically disposed relative to each other in a regularconfiguration, first and second ones of said windings being electrically connected in series and so disposed that current will flow through adjacent positions thereof in the same sense;

a pair of input terminals connected with the free ends of said series-connected first and second windings, respectively;

and a plurality of output terminals a first one of which is connected with the junction between said first and second windings and an additional pair of which are connected with the free ends of said series-connected first and second windings, respectively.

2. Apparatus as defined in claim 1, and further including a transformer core upon which the twisted windings are wound.

3. A potential divider according to claim 2 wherein said core is toroidal.

4. A potential divider according to claim 1, comprising a further transformer similar to the first said transformer. and means for connecting the input terminals of said further transformer to selected ones of the output terminals of the first said transformer.

' 5. A potential divider including a transformer which comprises at least three windings extending adjacent each other for mutual inductive coupling, two terminals each connected respectively to ends of first and second, ones of said windings, at least said first and second windings being connected in series between said terminals to provide at least one or more junctions between said windings and being disposed so that current flows through adjacent portions thereof between said input terminals in the same sense and at least one tap connected to said at least one junction, and negative impedance means connected across a third one of said windings to present to said third winding an impedance having a negative real part.

6. Potential divider .transformer means, comprising:

at least three mutually inductively coupled similar windings disposed symmetrically relative to each other in a regular configuration;

a pair of input terminals, first and second ones of said windings being connected in series between said input terminals and being so disposed that current flows through adjacent portions thereof in the same sense;

at least three output terminals, a pair of said output terminals being connected with said input terminals, respectively, and a third one of said output terminals being connected to the junction between said first and second windings;

and negative impedance means connected across said third winding to present thereto an impedance having a negative real part, whereby at least part of the iron and resistive losses in the transformer are cancelled.

References Cited UNITED STATES PATENTS 5/ 1969 Maxwell. 4/1958 Cutler et al 32343.5 X 4/1965 Keats 32343.5

US. Cl. X.R. 

